Activated rasG, rasG(G12T), was expressed in Dictyostelium cells under the
control of the folate-repressible discoidin promoter (pVEII-rasG(G12T)) and
found to have a unique pattern of expression when cells were transferred t
o folate-deficient media: an initial increase of RasG(G12T) resulting from
the removal of folate, followed by a rapid decline while cells were still i
n the early exponential phase of growth. Discoidin levels were considerably
lower and declined more rapidly in the pVEII-rasG(G12T) transformant than
they did in the wild type, suggesting that RasG(G12T) represses discoidin e
xpression. This was independently confirmed by placing the rasG(G12T) gene
under the control of the ribonucleotide reductase (rnrB) promoter. Exposure
of cells to 10 mM methyl methanesulfonate (MMS) rapidly generated RasG(G12
T) and this was accompanied by an equally rapid decrease in discoidin mRNA
levels. rasG null cells also contained decreased levels of discoidin under
all conditions tested, indicating that RasG is essential for optimum discoi
din expression. However, rasG null cells showed normal regulation of discoi
din expression in response to PSF, CMF, folate, bacteria, and axenic media,
indicating that RasG is not necessary for any of these responses. These re
sults reveal a role for RasG in regulating discoidin gene expression and ad
d a further level of complexity to the regulation of the discoidin promoter
. (C) 2001 Academic Press.